Formulation of a coating which is suitable for exterior use such as on the surfaces of an automobile is complex. The reason is that the coating must remain essentially unchanged in appearance over a period of several years while being exposed to a variety of weather conditions. The two major components of the coating are the vehicle and the pigment, and individually both can vary widely in stability properties.
Titanium dioxide is a most important pigment in such coatings and there is large quantity of literature on the methods and techniques to increase the stability properties of pigmentary titanium dioxide. Metal oxide-coated mica nacreous pigments such as titanium dioxide-coated mica, on the other hand, present a much more complicated entity than pigmentary titanium dioxide with respect to stability properties per se and to the coating exposed to the weather. Methods and techniques which are used to stabilize pigmentary titanium dioxide are either ineffective or insufficient to provide stability for titanium dioxide coated mica platelets.
The industry standard weatherstability testing is to subject coated metal panels to outdoor Florida weather for at least 2 or 3 years. The conditions prevalent there are most severe since the daily cycle includes the night with lower temperature and high humidity, possibly with some water condensation on the panels, a change to intense sunlight in the morning along with substantial temperature increases, the possibility of liquid water on the panel from rain in the afternoon followed by sunlight again and decreasing humidity, and finally the night again with falling temperatures and increasing humidity. This type of testing is especially common in connection with coatings intended for automotive use.
Since it takes such a long time to obtain meaningful outdoor exposure results, a number of accelerated weathering tests have been developed which can be completed in a much shorter period of time. These tests are used to screen potential candidates and determine whether the long term outdoor exposure testing should be conducted.
Three types of accelerated tests have evolved over the years. The first is a low temperature water immersion test (LTWI) where a pigment is incorporated in a paint system and then applied to a primed steel panel. The panel is then partially immersed in 35.degree.-50.degree. C. water for a week to ten days. After drying, changes in the immersed section of the panel relative to that part of the panel which was not immersed in the water are noted.
The second type of test commonly used is designed to evaluate the humidity or condensation resistance of the painted panel. A partially masked panel is placed in a condensation device such as the Cleveland Humidity Tester and subjected to 100-250 hours of condensation at 40.degree.-60.degree. C. At the end of the exposure period, changes in the exposed portion of the panel are compared to the unexposed portion of the panel which had been protected by the mask. Since the Cleveland tester is manufactured by the Q-Panel company, this test is commonly referred to as the Q-C-T test.
The third type of test involves exposing the panel to alternative cycles of UV radiation and condensation. Use is made of a laboratory instrument, the Q-U-V Accelerated Weathering Tester also made by Q-Panel, which provides cyclic weather conditions for coated metal panels so that in a 24-hour cycle, variations in near ultraviolet light, water condensation and temperature are presented to those panels. A typical Q-U-V cycle can be UV radiation for about 8 hours at 60.degree.-70.degree. C. followed by 4 hours of condensation at 50.degree.-60.degree. C. and the cycle is repeated over a period of 6 to 8 weeks. As in the other tests, changes in the exposed and unexposed portions of the panel are compared.
Many years of experience with these three accelerated tests have shown that products which fail any one of these tests will generally not pass the outdoor exposure testing. Unfortunately, experience has also demonstrated that the products which pass all three accelerated tests may not always pass the outdoor exposure testing. Because of this, some automotive paint suppliers and companies have begun to rely on an additional new accelerated test which must be satisfactorily completed before the outdoor exposure testing will be begun. This much more severe test involves immersing panels in 80.degree. C. water for 8 to 24 hours.
The initial treatments which were employed to stabilized pearlescent pigments for use in exterior applications involve the use of chromium. For example, U.S. Pat. No. 3,832,208 describes the use of methacrylatochromic chloride and U.S. Pat. No. 4,134,776 describes the use of chromium hydroxide. While such chromium treatments were satisfactory, there has been a movement away from the use of chromium in recent years because of the potential impact of chromium on the environment, the hazards of hexavalent chromium and its slightly greenish color. Accordingly, a demand developed for a non-chromium treatment for stabilizing pearlescent pigments. A number of non-chromium treatments were developed and provide products which are able to withstand the low temperature water immersion test, the Q-C-T condensation test and the Q-U-V radiation condensation test. Because of industry demands, these new non-chromium treatments must now also withstand the harshness of being immersed in 80.degree. C. water for an extended period of time in order to achieve sufficient acceptance by automotive paint companies to justify outdoor testing.
Canadian patent 664,268 which issued in 1963 discloses that the photoactivity of pigmentary rutile TiO.sub.2 pigments in plastic resins could be reduced by treating the pigmentary TiO.sub.2 with a combination of aluminum, cerium and silica. The patent notes that only a combination of the three components provides the stability increase. Data is set forth which shows that treatment of the calcined pigment with cerium alone, the combination of silicon and aluminum or silicon and cerium or aluminum and cerium resulted in degradation relative to untreated calcined pigment.
In the 1980s, U.S. Pat. Nos. 4,461,810 and 4,737,194 taught that pigmentary titanium dioxide coated with alumina could be stabilized with cerium provided sulphate, phosphate, silicate, borate or water soluble polyfunctional organic acid anions were also present.
U.S. Pat. No. 4,544,415 discloses pearlescent pigments based on metal oxide coated mica could have their weather resistance improved by coating the metal oxide with a top coat which contains a polysiloxane and a rare earth metal compound, preferably a compound of cerium. It was noted that the further addition of aluminum and zinc hydroxides reduced, in many cases, the tendency of the pigments to agglomerate and improved dispersability.
Published European patent application 342,533 relates to weather resistant pearlescent pigments in which metal oxide coated mica is overcoated with hydrated zirconium oxide and hydrated metal oxide in which the metal is cobalt, manganese or cerium. The published application indicates that additional stability can be obtained by adding hydrates, oxides or silicates of aluminum and/or zinc and that even better stabilities can be achieved by adding a siloxane coupling agent.
The object of the present invention is to provide new pearlescent pigments which do not contain chromium which can withstand not only the LTWI, Q-C-T and Q-U-V accelerated tests but can also withstand the harshness of the 80.degree. C. water immersion test. This and other objects of the invention will become apparent to those of ordinary skill in this art from the following detailed description.